Mobile stations of cellular and personal communications systems need to acquire certain cell-specific information before negotiating service with a serving base station. The base stations are strategically placed in a geographical area (based on capacity and coverage requirements), and each base station transmits the necessary cell-specific information. In spread-spectrum systems, the cell-specific information is carried by the pilot and/or synchronization channels. The synchronization channels are spread with pseudo-random or Gold code sequences, which form the basis for the mobile station's frame, slot and bit timing synchronization. The sequences also identify the base station. At power-up, the mobile station needs to search over the set of known system-specific codes to identify the serving base station and to achieve frequency, time and frame/slot synchronization.
Due to the combined presence of natural noise, the non-ideal characteristics of circuits in the mobile station, and other anomalies, search and synchronization processes are statistical in nature. The system designer must therefore optimize the design with respect to the following performance metrics: average (mean) and maximum search times; Pd, which is the probability of the correct identification of the serving base station, Pm, which is the probability of missed identification of the serving base station given a correct signal; and Pf, which is the probability of falsely identifying a base station that in fact is not visible to the mobile station.
To keep search times within practical limits, methods of searching over a portion of the pertinent code “spaces” are used in practice. A shortcoming with prior art approaches is that these methods result in a poor compromise among the above parameters. For example, for reasonably high Pd and low Pf, search times are undesirably high. Conversely, for reasonable search times, Pd may be less than desired, or Pf may be excessive.
Accordingly, what is needed is an improved search and synchronization method and apparatus that can achieve a low “miss” probability within a short search time.